R CMD check --as-cran sets _R_CHECK_LIMIT_CORES_, and under it
parallel::mclapply() does not quietly use fewer cores — it errors, via
parallel:::.check_ncores(), the moment more than two are asked for. So a
default of "cores minus one" turns every jackknife into a failure on any
machine with four or more cores, in exactly the context where a package is
most likely to be run by someone other than its author.